1. Field of the Invention
The present invention relates to transport refrigeration systems, and more particularly to a method for changing operational modes of transport refrigeration systems.
2. Description of Related Art
A particular difficulty of transporting perishable items is that such items must be maintained within a narrow temperature range to prevent, depending on the items, spoilage or conversely damage from freezing. In order to maintain proper temperatures within a transport cargo space temperature sensors are typically provided in a transport refrigeration unit that measure both the supply air and return air temperatures exiting and entering the unit, respectively. These sensors are usually housed directly within the refrigeration unit so that the sensors can be positioned during manufacture of the unit rather than having to be reinstalled each time a unit is moved to a new cargo container.
It has been recognized, however, that the temperature of the perishables within a cargo space does not necessarily correspond with either the temperature of the refrigeration unit's supply or return air. When such a unit is operated in an automatic start/stop mode for the purpose of conserving fuel by the unit's internal combustion engine, basing the unit's operational cycle on the supply air temperature may cause the unit to cease operation before the temperature within the perishable container is properly "pulled down" to a pre-determined setpoint temperature, thus leading to spoilage. Similarly, if the operational cycle is based upon the return air sensor then the perishables may "top-freeze" from too much supply air.
Transport refrigeration monitoring systems for use with perishable products are well known. Truckenbrod et al (U.S. Pat. No. 5,209,072) ("Truckenbrod") teaches a refrigeration control system having multiple sensors that varies the sensor being used to control the refrigeration unit based upon the interrelation of the sensors, and it is hereby incorporated by reference. The Truckenbrod system, however, is limited by both its complexity and inability to operate in a start/stop mode-that is, the engine used in association with Truckenbrod et al. is in continuous operation and therefore cannot enjoy the fuel savings of a start/stop operational mode. It would be desirable to have a transport refrigeration system that could run in a start/stop mode so as to enjoy fuel savings over running in a continuous operation mode while maintaining the integrity of perishable products within the transport container.